1. Field of the Invention
This invention relates to a technology of suppressing lowering of a temperature inside a purification apparatus due to a temperature difference between the apparatus-interior temperature and an apparatus-exterior temperature, and more particularly to a technology of suppressing polymerization of easily-polymerizable compounds on or around an inner wall of the purification apparatus.
2. Description of the Related Art
Vinyl compounds such as (meth)acrylic acid are known as easily polymerizable compounds. Particularly, in a purification process of (meth)acrylic acid and its ester in a purification apparatus such as a distillation column, such a compound is easily polymerized in the purification apparatus. Therefore, it is required to suspend the operation of the purification apparatus periodically to remove polymerized compounds.
One of the known techniques to prevent polymerization of easily-polymerizable compounds in the purification apparatus is to add a polymerization inhibitor such as hydroquinone to the purification apparatus. Another known technique to suppress adhesion of polymer on or around the inner wall of the apparatus is to apply a surface treatment that enhances smoothness on the inner wall of the apparatus such as disclosed in Japanese Patent No. 2633277.
Use of the polymerization inhibitor, however, cannot securely prevent polymerization in a condition in which (meth)acrylic acid is extremely likely to polymerize. For instance, in a case where a (meth)acrylic acid aqueous solution is subjected to distillation in an azeotropic distillation column (column for separating water component and crude (meth)acrylic acid), (meth)acrylic acid easily polymerizes in the azeotropic distillation column despite the use of a polymerization inhibitor. As the polymerization progresses, the distillation efficiency in the distillation column is gradually lowered, which necessitates suspending operation of the purification apparatus including the azeotropic distillation column to remove the polymer. Theoretically, it is possible to improve or at least secure a certain polymerization preventive effect by increasing the amount of a polymerization inhibitor to be added to the purification apparatus. However, such an attempt of preventing or suppressing polymerization has a limitation. In addition to this, gas in the distillation column does not contain a polymerization inhibitor, but rather contains an easily-polymerizable compound. Therefore, polymers unavoidably generate on or around the inner wall of the distillation column in contact with such an easily-polymerizable-compound-containing gas (hereinafter, the area of the inner wall in contact with the gas is sometimes referred to as “gas-phase contact area”).
Japanese Examined Patent Publication No. 63-11921 discloses a method of preventing polymerization of a condensate even in a condition in which substantially no polymerization inhibitor exists in the distillation column. In this method, a heating means (e.g., heating jacket) is mounted or covered over the distillation column to prevent condensation of vapors on or around the gas-phase contact area of the column, thus preventing polymerization of the condensate. This method requires heating the purification apparatus from the outside. This method is infeasible particularly in a case of mounting a heating jacket over a large-scaled distillation column because the facility cost is expensive, and the temperature control inside the distillation column is difficult.
There has also been proposed an idea of providing a heating means such as a heating jacket on the outer wall of the distillation column and passing a heating medium such as warm water, hot water, and steam inside the heating jacket to prevent condensation of the easily-polymerizable-compound-containing gas on or around the gas-phase contact area of the column so as to suppress generation of polymer on or around the gas-phase contact area. However, the idea of providing a heating jacket in the above manner involves a drawback in that excessive heating leads to unstable running of the distillation column. Further, the above idea is not desirable with respect to safety measures because once the heating jacket causes breakage or damage, the heating medium running in the heating jacket is likely to flow or come outside. Particularly, the idea of passing a heating medium inside a heating jacket may encounter the following difficulty. When the above technology of providing a heating jacket and passing a heating medium inside the heating jacket is applied to the distillation column equipped with an attachment, the heating jacket is required to be mounted over the distillation column in such a manner as to avoid the region where the attachment is mounted because of the fact that passing the heating medium inside the heating jacket mounted over the outer wall of the distillation column including the attachment mounted region hinders efficient heating of the column outer wall. Therefore, in case of mounting a heating jacket over the distillation column directly equipped with an attachment on the outer wall thereof, the installed location of the heating jacket is restricted by the existence of the attachment to a location where an attachment is less likely to be mounted, such as an upper part or a lower part of the distillation column. The restricted installation of the heating jacket cannot sufficiently prevent generation of a polymer on or around the gas-phase contact area and/or adhesion of such polymer thereto. In such an arrangement, it is highly likely that a polymer will generate and/or adhere on or around the inner wall region (gas-phase contact area) of the column corresponding to the site where the heating jacket is not mounted.
Liquids and gases are fed in and out of the distillation column through pipes. Specifically, taking an example of a liquid, the liquid is fed out of the distillation column and is drawn into a reboiler where the liquid is heated, and the heated liquid is fed back to the distillation column. At the time of circulating the liquid through a reflux pipe from the reboiler to the distillation column, there is a possibility that the liquid temperature may be lowered in the course of circulating. Heat insulating means such as a heating jacket is covered over the circulation pipe to prevent such a temperature lowering. Similar to the heat insulating means mounted on the reflux pipe, heat insulating means such as a heating jacket and a heat insulator is mounted over a gas-phase contacting pipe which connects the reboiler and the distillation column, or over a nozzle connecting a communication port of the distillation column with a communication port of the reboiler. This arrangement is effective in suppressing polymerization of the easily-polymerizable compound contained in the gas on or around the pipe or the nozzle by preventing condensation of the gas on or around the pipe or the nozzle. However, it is not effective in preventing generation of a polymer with respect to the whole interior of the distillation column.